1. Field of the Invention
The invention relates to digital controllers that are used to control industrial machines and processes.
2. Description of the Prior Art
A programmable controller for controlling industrial machines and processes typically has a main processor and a group of input and output (I/O) interface modules. The I/O modules are mounted in the slots of an equipment rack. A slot at the left end can accommodate a small processor module. This slot can also accommodate an adapter module when the main processor is located apart from the equipment racks. These two alternative processor arrangements are shown and described in Struger, U.S. Pat. No. 4,250,563, issued Feb. 10, 1981. There, the separate main processor is located relatively close to the equipment rack, but with the use of a serial data link, the main processor can be located a great distance away as described in Schultz et al, U.S. Pat. No. 4,413,319, issued Nov. 1, 1983.
An I/O module contains either a plurality of output circuits or a plurality of input circuits. The circuits are connected through edge connectors on the front of the rack and through bundles of external wires to input and output devices on a machine or process. Typical output devices are solenoids, relays and motor starters. Typical input devices are limit switches, photoelectric sensors and proximity sensors. Each single-bit output circuit generates the proper AC or DC signal to operate an output device in response to a single digital data signal. Each single-bit input circuit responds to an AC or DC signal from an input device to generate an individual digital data signal. By including eight discrete input circuits and some multiplexing circuitry in a single input module, data is collected from individual I/O devices and assembled into an eight-bit word of data or "byte". The byte can then be transmitted over a single data channel. Similarly, by including eight discrete output circuits and some demultiplexing circuitry in a single output module, signals can be distributed from bytes of data to individual output devices.
A controller of an earlier generation, shown in Dummermuth, U.S. Pat. No. 3,942,158, issued Mar. 2, 1976, includes sixteen input circuits or sixteen output circuits per I/O module, as well as a sixteen-bit I/O data bus. In following generations of controllers, the size and amount of hardware in the I/O modules and the equipment rack supporting them was reduced by a factor of about one half, by reducing the number of circuits per module to eight and by using an eight-bit data bus. It is now desirable to increase the number of I/O circuits per module to sixteen while maintaining the package size now being used for modules with eight I/O circuits. This increase in the density of I/O circuits is to be accompanied at a reduced cost per I/O bit of capacity.
From a system viewpoint, the I/O modules are connected on one side (the back) to the main processor and on another side (the front) to machine or process control devices. From an electrical viewpoint, the I/O modules isolate 120-volt AC signals or 24-volt DC signals, for example, on the machine side of the controller, from the 5-volt logic level signals used by the main processor. This electrical isolation is typically accomplished with optically coupled circuits in which current on an input side causes an emitter to emit light. The light is received by a light detector that generates an output current. With optical coupling, an overcurrent or fault on the machine side of the I/O module will not cause a corresponding overcurrent or fault on the processor side.
The conventional practice is to allocate one optically coupled circuit for each single-bit I/O device, so that all of the I/O devices are isolated from the main processor. An increase in I/O circuits per module would normally result in an increase in such optically coupled circuits. In the present state of the art, these optically coupled circuits are more expensive and are more sensitive to heat than the other types of integrated circuits used in I/O modules. Therefore, the increase in density of I/O circuits is not easily achieved.
Another technical problem is maintaining the compatability of new I/O modules with presently available processors and equipment racks that are the other parts of the modular system. These processors use a scanning technique based on a single-byte transfer to or from each eight-bit I/O module, so the equipment rack has a backplane circuit board that uses a byte-wide data bus. If each I/O module is to handle two bytes, there must be an improvement in the enabling circuitry to allow the two bytes to be transferred over the single-byte data bus.